1. Field of the Invention
The present invention relates to an electric power steering apparatus using a brushless DC motor for steering assistance. More specifically, the present invention relates to the electric power steering apparatus which comprises an angle detector (angle sensor) for outputting a sine wave signal and a cosine wave signal which correspond to a rotor position of a brushless DC motor and a torque detector for detecting a steering torque applied to a steering member, and drives the brushless DC motor, based upon the sine wave signal and the cosine wave signal which were outputted from the angle detector and the steering torque detected by the torque detector, to assist steering. Further, the present invention relates to a method for detecting abnormality of an angle detector for use in such electric power steering apparatus.
2. Description of Related Art
An electric power steering apparatus, which drives an electric motor for assisting steering so as to alleviate the load of a driver, typically comprises: an input shaft connected to a steering member (steering wheel); an output shaft connected to steering control wheels through a pinion, rack and the like; and a connecting shaft for connecting the input shaft and the output shaft. A steering torque value to be applied to the input shaft is detected by a torque sensor based upon an angle of distortion generated in the connecting shaft, and the electric motor for steering assistance, which is coupled with the output shaft, is driven and controlled.
In recent years, a brushless DC motor has been in use in such an electric power steering apparatus as described above. In the brushless DC motor of the electric power steering apparatus, a current flowing through a stator is PWM (pulse width modulation) controlled such that a rotary magnetic field generates according to a rotating position of a rotor. For detecting the rotor position, an angle sensor (angle detector) such as a resolver or an MR sensor for outputting a sine wave signal and a cosine wave signal is used.
Incidentally, when the angle sensor (angle detector) used for detecting the rotor position is broken down and thereby an accurate data of the rotating position cannot be obtained, it is not possible to allow an appropriate current according to the rotor position to flow through the brushless DC motor. In such case, therefore, a steering wheel (steering member) might vibrate or be locked in the electric power steering apparatus using the brushless DC motor. In such circumstances, the following inventions have been proposed with the aim of detecting abnormality in the brushless DC motor, more specifically, in the angle sensor (angle detector).
Japanese Patent Application Laid-Open No. 2002-81961 discloses an invention in which an analog signal of a magnetic detection element is converted to a digital signal, and according to a difference value between the digital signal and a resolver signal, abnormality in an angle sensor is detected.
Japanese Patent Application Laid-Open No. 10-278826 (1998) discloses an invention in which a steering angle given from an angle sensor at a predetermined cycle is stored, a derivation value of the steering angle is calculated, an absolute value of a difference from a previous derivation value is obtained, and the obtained absolute value is compared with a threshold, to detect abnormality.
Japanese Patent Application Laid-Open No. 10-258757 (1998) discloses an invention in which a difference between two optional outputs among outputs of three respective steering angle sensors is obtained and it is determined that an abnormal sensor is included in a combination of the two sensors whose output difference is a value apart from zero.
Japanese Patent Application Laid-Open No. 10-197238 (1998) discloses an invention in which a predetermined number of samplings of a steering angle having been detected by a steering angle sensor are performed within a predetermined period of time to calculate an average and a derivation, and when the average or the derivation falls out of a predetermined range, the sensor is determined as abnormal.
However, there has been a problem in that abnormality in a detector cannot be detected at an early stage in the case of the above-mentioned prior arts.